For a dipped beam, such a cutoff or masking limit may, for example, be constituted by two half planes passing through the optical axis of the headlamp such that for right-hand drive the left half plane is substantially horizontal and the right half plane is slightly inclined to the horizontal. This arrangement is defined in the French Highway Code and in the European standards laid down in the Geneva regulations. In other countries, for example in the United States, sealed beam type headlamps are masked by means of two half planes which are horizontal and which extend on either side of the light axis. "Z-masked" beams are also known.
More precisely, the present invention relates to masked beam headlamps of the type comprising a reflector, a light source, a masking screen provided with a masking edge, and a converging lens. In a typical embodiment:
the reflector is at least partially in the form of an ellipsoidal cap which is circularly symmetrical about an axis defining an optical axis, with the two focuses of the ellipsoid defining a first focus close to the reflector and a second focus further away therefrom;
the light source is disposed in the vicinity of the first focus of the reflector;
the converging lens is disposed with its optical axis coinciding with the optical axis of the reflector, with its focus in the vicinity of the second focus of the reflector, and on the opposite side of said second focus to the reflector; and
the masking screen is disposed in the vicinity of the lens focus and its masking edge is close to the optical axis.
Such a structure has been known for a long time, and is described, for example, in British patent certification No. 450 348 which was published in 1936.
In practice, such optical systems have been very little used for motor vehicle headlamps, and the Applicant has recently taken an interest in such systems.
Generally speaking, the Applicant has observed that simply implementing the teaching of the prior art as it stands does not enable a headlamp to be obtained capable of satisfying modern requirements of motor vehicle manufacturers and modern regulatory standards.
The Applicant has observed that the essential problem lies in the way the masking screen is made.
In the prior art these screens are simply plane or substantially plane opaque masks of uniform thickness whose top edges define a masking or cutoff limit.
Tests performed by the Applicant have shown that such a screen in a headlamp of the above-mentioned type is inherently subjected to two serious defects: in brief, the system provides too sharp a cutoff, and provides too little light above the cutoff limit.
The first drawback is particularly apparent when the driver of a vehicle fitted with such dipped headlamps brakes suddenly. In this case, the vehicle tips forwardly and the distance of visibility provided by the headlamps is suddenly reduced to a few meters. By virtue of the total absence of any light above the cutoff limit, the driver has the impression of a "black hole" and is subjected to the well-known risks of such a situation.
Further, the absence of any light above the cutoff limit makes it impossible to read some kinds of road sign.
Finally, simple opaque masking thus turns out to unacceptable in practice for making a headlamp which satisfies French and European standards.
The same is true of headlamps for American standards, and in particular for masked beam standards applicable to "sealed beam" type lamps which do not allow for such a sharply marked cutoff.
The problem is thus to obtain a satisfactorily masked beam with headlamps of the above-defined structure. The invention provides a set of solutions to this problem.